Anodization is an electrolytic passivation process that increases the thickness of the natural oxide layer on the surface of an aluminum or aluminum alloy component. The process is used to improve resistance to corrosion and wear. Paints, primers, coatings, and adhesives adhere better to anodized aluminum and anodized aluminum alloy surfaces than to untreated surfaces. Another advantage of anodization is that relatively thick porous surfaces can be developed which are capable of absorbing dyes to provide a variety of different colors.
The porous surface developed by anodization creates a vulnerability to corrosion. To reduce this vulnerability to corrosion, a sealing process is often employed. Sealing can also be used to improve dye retention.
In preparation of aluminum or aluminum alloy articles having an anodized surface, the sealing step is often the most time-consuming, energy intensive, and expensive step. In accordance with the various embodiments of the invention, one or a combination of two or more of these issues are addressed.
Currently, there are a large variety of sealing methods commercially employed and described in the patent literature.
The most basic type of sealing involves simply immersing the anodically treated aluminum or aluminum alloy article in water at or near the boiling temperature. The hot water dissolves material from sidewalls of pores in the oxide layer. This material is hydrated to form boehmite, aluminum hydroxide gel, pseudoboehmite, and crystalline boehmite. These various hydrated products then swell and become less water soluble to partially close or seal the pores. This method is inefficient with respect to both time and energy. It typically takes about an hour to properly seal and anodically treated aluminum or aluminum alloy having an oxide film layer thickness of about 0.6 mils, and the energy required to maintain water at or near the boiling temperature is substantial. In addition, material from the oxide layer is destroyed, making the oxide layer softer and less resistant to abrasion.
Other methods of sealing aluminum oxide layers formed by an anodic treatment have been developed. The most popular of these methods involves immersing the anodically treated aluminum article into a solution of hot nickel acetate. The hot water dissolves material from the sidewalls of the pores in the oxide layer, and the dissolved material reacts with nickel ions to form a solid. The time required to seal a 0.6 mil thick oxide layer using this method is about 30 minutes (i.e., about half the time of using hot water at or near the boiling temperature). However, this still represents a very time-consuming and energy intensive process that causes damage to the oxide layer resulting in reduced hardness and reduced abrasion resistance.